CP1342: Operating Systems - Lecture 9: Deadlock Characterization - BCA S3

In a deadlock, processes never finish executing, and system resources are tied up, preventing other jobs from starting.
Necessary Conditions
A deadlock situation can arise if the following four conditions hold simultaneously in a system:

1. Mutual exclusion. At least one resource must be held in a nonsharable mode; that is, only one process at a time can use the resource. If another process requests that resource, the requesting process must be delayed until the resource has been released.

2. Hold and wait. A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes.


3. No preemption. Resources cannot be preempted; that is, a resource can be released only voluntarily by the process holding it, after that process has completed its task.


4. Circular wait. A set {P0, P1, ..., Pn} of waiting processes must exist such that P0 is waiting for a resource held by P1, P1 is waiting for a resource
held by P2, ..., Pn−1 is waiting for a resource held by Pn, and Pn is waiting for a resource held by P0.


The circular-wait condition implies the hold-and-wait condition, so the four conditions are not completely independent.